Some multi-band or other tactical radios operate in the high frequency (HF), very high frequency (VHF) (for satellite communications), and ultra high frequency (UHF) bands. The range of these multi-band tactical radios can operate over about 2 through about 512 MHz frequency range. Next generation radios will probably cover about 2.0 to about 2,000 MHz (or higher) to accommodate high data rate waveforms and less crowded frequency bands. This high frequency transmit mode is governed by standards such as MIL-STD-188-141B, while data modulation/demodulation is governed by standards such as MIL-STD-188-110B, the disclosures which are incorporated by reference in their entirety.
UHF standards, on the other hand, provide different challenges over the 225 to about 512 MHz frequency range, including short-haul line-of-sight (LOS) communication and satellite communications (SATCOM) and cable. This type of propagation can be obtained through different weather conditions, foliage and other obstacles making UHF SATCOM an indispensable communications medium for many agencies. Different directional antennas can be used to improve antenna gain and improve data rates on the transmit and receive links. This type of communication is typically governed in one example by MIL-STD-188-181B, the disclosure which is incorporated by reference in its entirety. This standard specifies a family of constant and non-constant amplitude waveforms for use over satellite links.
The joint tactical radio system (JTRS) implements some of these standards and has different designs that use oscillators, mixers, switchers, splitters, combiners and power amplifier devices to cover different frequency ranges. The modulation schemes used for these types of systems can occupy a fixed bandwidth channel at a fixed carrier frequency or can be frequency-hopped. These systems usually utilize memoryless modulations, such as a phase shift keying (PSK), amplitude shift keying (ASK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), or modulations with memory such as continuous phase modulation (CPM) and combine them with a convolutional or other type of forward error correction code.
Minimum shift keying (MSK) and Gaussian minimum shift keying (GSMK), together referred to as MSK or GMSK, are a form of frequency shift keying (FSK) used in the Global System for Mobile communications (GSM). The circuits used for implementing such waveforms could include a continuous phase frequency shift keying (FSK) modulator.
Briefly, an MSK modulated signal can be considered as two combined orthogonal signals or channels that are 90 degrees out of phase with each other. Typically, each phase reversal is keyed to represent alternate bits of a binary signal that is to be transmitted. Each keyed pulse period could have a duration of a two bit period that is staggered by a one bit period, and when each channel is phase-shift keyed, it can be amplitude modulated with a one-half sinusoid and combined by addition. Because the sine shaped envelopes of the two channels are 90 degrees out of phase with each other, the sum of the two channels results in a signal with a constant envelope amplitude, which could be amplified by non-linear class-C amplifiers and transmitted. A Gaussian filter having a Gaussian impulse response can be used for prefiltering symbols prior to any continuous phase modulation, thus allowing a Gaussian minimum shift keying.
Many of the radio frequency (RF) power amplifiers used in these communications systems having MSK or GMSK modulation are peak power limited. For example, average power transmitted can be several decibels (dB) less than the peak power capability of an RF amplifier because of the back-off required for the waveform peak-to-average ratio. As a result, a constant amplitude waveform is necessary to address this issue.
A problem encountered in communications systems that use MSK or GMSK modulation is how the systems cope with multipath fading environments when the multipath encountered extends over many symbols. Fading is often caused by reflections and waveform distortion effects caused by variations in signal propagation. MSK or GMSK systems typically use a maximum likelihood sequence estimator (MLSE), also commonly referred to as a Viterbi equalizer, to handle multipath. Unfortunately, the computational complexity of this type of equalizer grows exponentially with the length of the channel. A requirement exists in industry to design and develop constant amplitude waveforms with more multipath capability than could be afforded by Viterbi or MSLE equalizers.